This invention relates to the measurement and control of magnetomotive forces that influence magnetic fluxes in magnetic bodies. When applied to current transformers, the invention relates to the measurement of electric current, especially electric current that has a d-c (direct-current) component.
Magnetomotive force (F) is associated with the art of magnetic circuits, and is often defined for a closed loop as the line integral of the magnetic field strength (H) around the closed loop:F=H ·dl Magnetomotive force F is a scalar quantity associated with the closed loop, while magnetic field strength H is a vector quantity. By Ampere's Law, magnetomotive force is proportional to the total current flowing through the closed loop. Utilizing the meter-kilogram-second (m.k.s.) system of units, magnetomotive force has units of amperes (or amp-turns), and is equal to the total current flowing through the closed loop. The closed loop is often chosen to pass through one or more conductive windings wrapped around a magnetic body, and a magnetomotive force equal to the current in the winding times the number of winding turns is associated with each winding (thus the unit “amp-turns”). The total magnetomotive force for the closed loop is the sum of all amp-turn contributions from all windings and other conductors.
Many magnetic devices operate best with an average magnetomotive force near zero. A deviation away from zero often results in excessive buildup of magnetic flux that causes the device to malfunction. Ordinary current transformers are one type of device for which this is usually the case. Since an understanding of current transformer operation is important to understanding some embodiments of the invention, the background of current transformers will be discussed in some detail. However, it should be kept in mind that the invention is applicable to many different kinds of magnetic devices, and is not limited to current measurement applications.
Most current monitoring systems for a-c (alternating-current) electric power systems utilize ordinary current transformers to provide input currents that are isolated from the electric power system conductors, similar to FIG. 1. A current-carrying conductor 4 is configured as a primary winding of a current transformer CT1, and is magnetically coupled to a magnetic core 1. For clarification, the term “magnetic core” as used herein refers to a magnetic body having a defined relationship with one or more conductive windings. A secondary winding 2 is also magnetically coupled to magnetic core 1. The phrase “magnetically coupled” is intended to mean that flux changes in a magnetic body are associated with an induced voltage in the winding, the induced voltage being proportional to the rate of change of magnetic flux that is coupled, in accordance with Faraday's Law.
A secondary electric current J2 is induced in the secondary winding that is proportional to a primary electric current J1. The secondary current is isolated from the primary current and is smaller than the primary current by the turns ratio of the primary and secondary windings. The primary winding may consist of only one turn (as in FIG. 1) or may have multiple turns wrapped around the magnetic core. The secondary winding usually consists of multiple turns wrapped around the magnetic core.
The accuracy of a current transformer is usually related to the coercive force of the magnetic core material (less is better), the cross sectional area of the magnetic core (bigger is better), the effective magnetic length of the magnetic core (shorter is better), any air gap in the magnetic core (less or none is better), and the “squareness” of the magnetic core material hysteresis curve (squarer may be preferred if not operating near saturation, otherwise characteristics that are not square may be preferred). Split-core current transformer cores generally have hysteresis curves that are less square than standard current transformer cores due to the small air gaps inherent in the design of split-core current transformers.
In order for the secondary current generated by a current transformer to be an accurate representation of the primary current, the impedance of the circuit connected to the secondary winding must be kept low so that current can flow freely. The impedance of the secondary circuit is often called the “burden.” The burden generally includes all impedances in the loop through which the secondary current flows, including stray winding impedances, stray impedances of connecting conductors, and the impedances of any devices connected in the loop (such as current-sensing resistors and relay operating coils). In order for a current transformer to drive a secondary current through a non-zero burden, a voltage must be induced in the secondary winding. The induced voltage is proportional to secondary current and is proportional to the burden, in accordance with Ohm's Law (the induced voltage equals the secondary current times the vector sum of all secondary loop impedances). The induced voltage is induced in the secondary winding by a fluctuating magnetic flux in the magnetic core (the instantaneous magnitude of induced voltage being proportional to the rate of change of magnetic flux, in accordance with Faraday's Law). The fluctuating magnetic flux is associated with an “exciting current” in accordance with well-known electromagnetic principles. The exciting current is often understood to have a magnetizing component and a core loss component. When utilized to measure alternating current with no d-c component, the exciting current accounts for the error in the secondary current, and may be referred to herein as an “exciting current error.” Generally speaking, the accuracy of a current transformer is inversely related to the burden of the secondary circuit. A higher burden causes the current transformer to operate with greater induced voltage, thereby increasing the exciting current error, thereby causing the secondary current to be less accurately proportional to the primary current.
With preferred current transformer operation, the amp-turns of the primary winding are largely canceled by the amp-turns of the secondary winding, so that the magnetomotive force acting on the current transformer core is relatively small. The net magnetomotive force acting on the core is equal to the difference in amp-turns of the primary winding and the secondary winding, and this difference is proportional to a secondary current error.
Speaking more precisely of current transformer operation, a secondary electric current error is proportional to the magnetomotive force acting on the magnetic core. The instantaneous value of the magnetomotive force is equal to the instantaneous difference between the primary electric current multiplied by the number of turns of the primary winding and the secondary electric current multiplied by the number of turns of the secondary winding. The secondary electric current error comprises a d-c component and an a-c component; the d-c component will be referred to as a d-c current error, and the a-c component will be referred to as an exciting current error.
Ordinary current transformers work properly only with alternating primary current. When a d-c component is present in the primary current, normal current transformer operation cannot maintain a d-c component in the secondary circuit, and a large d-c current error results. This d-c current error correlates to a large d-c magnetomotive force applied to the magnetic core, which causes the magnetic core to saturate, thereby adversely affecting current transformer operation.
A great many variations to the basic current transformer circuit have been developed in the prior art to improve current transformer accuracy for various applications. Some of these are summarized here:                (a) Utilize an active load to sense current. An active load can have an effective burden of virtually zero Ohms, but this does not solve the problem of stray impedances contributing to the burden of the secondary circuit. The use of an active load to reduce current transformer burden is described in detail in U.S. Pat. No. Re. 28,851 to Milkovic (reissued 1976) for a “Current Transformer with Active Load Termination.”        (b) FIG. 2 illustrates one form of a prior-art “zero-flux” current transformer. A sense winding 10 terminated in a high-impedance manner provides a voltage signal V4 that is proportional to the rate of change of magnetic flux. By amplifying this signal and applying it in series with the secondary winding, the effective burden of the entire secondary circuit is reduced to near zero ohms. Magnetic flux oscillations in the current transformer core are reduced to near zero, and the exciting current required is reduced to near zero, thereby making secondary current more accurately proportional to primary current. The amplifier essentially provides the driving voltage necessary to drive loop current through secondary loop impedances so that the current transformer core does not need to generate this voltage via a changing flux. Higher gains in the amplifier circuit contribute to increased accuracy and smaller flux changes, though excessively high gain typically leads to instability and associated oscillations. This device provides very good accuracy for measurement of a-c current, but it cannot measure d-c current.        (c) In order to measure d-c current (or combined a-c and d-c current), Hall-effect current sensors are often used. These sensors typically insert a Hall-effect magnetic field sensor in a current transformer core air gap. In “open loop” devices, the magnetic field strength is used to estimate the primary current directly. “Closed loop” devices utilize a zero-flux concept similar to that described for FIG. 2. However, instead of using a sense winding (as in FIG. 2), the Hall-effect element generates a voltage signal proportional to the magnetic field in the air gap. A high-gain amplifier circuit is used to drive secondary current to continuously nullify the magnetic field, which causes the secondary winding amp-turns to balance the primary winding amp-turns. This results in a secondary current that is proportional to the primary current. A current-sensing resistor in the secondary circuit normally provides a voltage signal that is proportional to secondary current. While these Hall-effect current sensors are widely used, their accuracy and stability over time are not adequate for many applications.        (d) FIG. 3 shows another prior-art circuit that operates in a near-zero-flux manner. This type of “burden-reducing” circuit is described in U.S. patent application Ser. No. 09/713,921, filing date Nov. 15, 2000, by Edel. This patent application in its entirety is hereby incorporated by reference into this disclosure.         The circuit shown in FIG. 3 uses the secondary current as an input to generate the compensation voltage required to drive secondary current. This circuit has the advantage of utilizing ordinary current transformers without the need for a sense winding or Hall-effect sensor. However, the circuit shown in FIG. 3 can only be used to measure a-c current, and it is difficult to compensate for secondary loop impedance changes due to temperature changes. The associated patent describes how the control circuit can be modified to enable accurate measurement of d-c current, but the method used is dependent on brief periodic reset pulses applied to the magnetic core, during which time current cannot be measured.        (e) Many specialized current transformers with multiple windings and/or multiple cores have been developed. Many of these transformers have excellent accuracy. However, most of these specialized transformers are prohibitively expensive for many applications. Some devices having simple magnetic cores drive the core in and out of saturation to measure d-c current, often causing excessive noise on the primary circuit.        
It is therefore an object of the present invention to provide an economical current sensor with the following properties:                (a) Utilize an ordinary current transformer core.        (b) Provide for continuous measurement of a-c and d-c current.        (c) Have a high degree of stability over time and temperature.        (d) Have better accuracy than Hall-effect current sensors.        (e) Cause very little noise on the primary circuit.        
Another object of the invention is to provide a way to measure magnetomotive force experienced by a magnetic body without utilizing a Hall-effect sensor. Other objects will become apparent from the description of the invention.